Inverters are often used to convert DC power into AC power for one or more AC loads. For example, in a variable-speed, constant-frequency (VSCF) power conversion system, a brushless, synchronous generator converts variable-speed motive power produced by a prime mover into variable-frequency AC power. A rectifier converts the variable-frequency AC power into DC power and an inverter converts the DC power into constant-frequency AC power. The inverter may be of the stepped-waveform type that produces phase output waveforms each comprising a series of steps or levels. Typically, each phase output waveform is subdivided into six or a multiple of six conduction periods in each cycle of the output wherein the waveform assumes one of a plurality of constant levels in each conduction period.
Prior stepped-waveform inverters have utilized a plurality of three-phase subinverters coupled to a corresponding number of sets of primary windings of a summing transformer. Each set includes three phase windings connected together in either a delta or wye configuration. A secondary winding is inductively linked with each primary winding to form a plurality of sets of three-phase secondary windings. The secondary windings of the same phase are connected together in series to form three sets of series-connected windings. The series-connected windings are, in turn, connected together in a wye configuration.
The subinverters are operated to produce rectangular voltage waveforms that are applied to the sets of primary windings. Corresponding voltages are induced in the secondary windings and the voltages are summed owing to the series connection of the secondary windings to produce three-phase stepped-waveform output voltages. The output voltage of each phase comprises a 12-, 24- or 36-step waveform when two, four or six subinverters are used, respectively. A 24-step inverter of the above-described type is disclosed in Compoly et al., U.S. Pat. No. 3,775,662.
Stepped-waveform inverters produce harmonics that are more easily filtered than the harmonics present in a pulse-width modulated (PWM) output. Thus, the size and weight of a filter coupled to the inverter output may be reduced as compared with a PWM type of inverter. However, the summing transformer contributes significantly to the size and weight of the stepped-waveform inverter, and hence the decrease in size and weight in output filter is partially or fully offset by the increased size and weight of the inverter itself.